(a) Field of the Invention
The present invention relates to an IPS (in-plane-switching-mode) LCD (liquid crystal display) device and, more particularly, to suppression of the chromaticity shift between the normal viewing direction and a slanted viewing direction in the IPS LCD device.
(b) Description of the Related Art
An IPS LCD device uses a lateral electric field which rotates the orientations of the LC molecules of the LC layer in the direction parallel to the surfaces of the substrates. The IPS LCD device has the advantage of a wider viewing angle compared to a twisted-nematic-mode LCD device. FIG. 5 shows a typical IPS LCD device in a sectional view thereof.
The IPS LCD device, generally designated by numeral 200, includes a polarizing film 201, a TFT (thin-film-transistor) substrate 202, an LC (liquid crystal) layer 203, a CF (color-filter) substrate 204 and a polarizing film 205, consecutively arranged in the travelling direction of the backlight. In the IPS LCD device 200, an electric field parallel to the substrates 202 and 204 is applied to the LC molecules 210 in the LC layer 203 by a potential difference between a pixel electrode 208 and a counter electrode 209 in each pixel, for driving the LC molecules 210.
In general, although the IPS LCD device 200 has the advantage of the wider viewing angle, as described above, it is known that leakage light is observed upon display of, for example, black, as viewed in a slanted viewing direction from an azimuth angle of 45 degrees with respect to the polarizing axis of the polarizing film 201 or 205. This leakage light causes an undesirable chromaticity shift wherein the original color of black is shifted toward red. The leakage light is generated due to the facts that the angle between the polarizing axes of the polarizing films 201 and 205, which is actually 90 degrees, is observed more than 90 degrees as viewed from the azimuth angle of 45 degrees with respect to the polarizing axis of the polarizing film 201 or 205, that the light passing through the protective layers of the polarizing films 201 and 205 is observed as elliptically polarized light and thus has a retardation, and that the LC layer 203 has a birefringence as viewed in the slanted viewing direction.
FIG. 6 shows the relationship between the viewing angle with respect to the IPS LCD device upon display of black and the chromaticity observed, wherein the chromaticity shift caused by the change of the viewing angle is represented by the shift of the color temperature. The viewing angle is generally defined by an angle between the viewing direction and a normal line of the screen of the LCD device. In general, a color temperature of 1000 K corresponds to red, a color temperature between 2800 K and 3200 K corresponds to orange to yellow, a color temperature between 3200 K and 5000 K corresponds to achromatic or colorless, and a color temperature over 5000 K corresponds to blue.
If the chromaticity observed in the normal viewing direction, i.e., from a viewing angle of zero degree, with respect to the screen upon display of black corresponds to a color temperature of around 11000 K, a slanted viewing angle with respect to the normal line of the screen lowers the color temperature. In particular, the increase of the viewing angle abruptly lowers the color temperature, as shown in FIG. 6. The reduction of the color temperature means an increase of the red component in the light emitted from the LCD device.
The leakage light in a slanted viewing direction as well as the chromaticity shift observed between the normal viewing direction and a slanted viewing direction degrades the image quality of the LCD device, and thus is undesirable. Techniques for reducing the leakage light in the slanted viewing direction and suppressing the chromaticity shift are described in Patent Publications JP-A-10(1998)-307291 and 2001-242462. These techniques use optical compensation layers each having a specific optical characteristic to reduce the leakage light in the slanted viewing direction and suppress the chromaticity shift.
In the typical LCD device 200 shown in FIG. 5, a SiNx (insulation) layer 207 having a refractive index of around 2 is sandwiched between a glass substrate body 206 having a refractive index of 1.54, for example, and an LC layer 203 having a refractive index of around 1.5. In such a case, wherein a layer having a higher refractive index is sandwiched between a pair of layers having a lower refractive index (or refractive indexes), the wavelength characteristic of the transmittance of the light is changed, due to the interference of light, depending on the thickness of the layer sandwiched between the layers having the lower refractive index.
In the case of the LCD device 200, the insulation layer 207 has an apparent thickness (d′) as observed in the slanted viewing direction, which is different from the thickness (d) thereof as observed in the normal viewing direction. Due to this apparent difference in the thickness of the insulation layer 207, the insulation layer 207 has a difference in the wavelength characteristic of the transmittance between the slanted viewing direction and the normal viewing direction.
It is assumed here that a 3-wavelength light source having respective peaks in three wavelength ranges corresponding to red, green and blue is used as the backlight source, and that the wavelength characteristic of the transmittance as observed in the normal viewing direction is such that the transmittances of the light components having the wavelengths corresponding to the three peaks are not reduced. In such a case, if the wavelength characteristic of the transmittance in the slanted viewing direction is such that at least one of the peeks in the three wavelength ranges is reduced, then a significant difference arises in the wavelength spectrum between the light transmitted in the slanted viewing direction and the light transmitted in the normal viewing direction. This difference causes a significant chromaticity shift between the slanted viewing direction and the normal viewing direction. In the conventional techniques such as described in the above patent publications, there is no suggestion for determining the thickness of the insulation layer 207 in consideration of the difference of the wavelength characteristic of transmittance between the normal viewing direction and the slanted viewing direction.
In view of the above problems in the conventional techniques, it is an object of the present invention to provide an IPS LCD device which is capable of reducing the leakage light in the slanted viewing direction and suppressing the chromaticity shift between the normal viewing direction and the slanted viewing direction, thereby improving the image quality of the IPS LCD device.
The present invention provides an IPS LCD device including: a liquid crystal (LC) layer having a first refractive index; a light-incident-side substrate and a light-emitting-side substrate sandwiching therebetween the LC layer; and a light-incident-side polarizing film and a light-emitting-side polarizing film sandwiching therebetween the substrates and the LC layer, the light-incident-side substrate including a substrate body having a second refractive index and an insulation layer overlying the substrate body and having a third refractive index which is higher than the first and second refractive indexes, the insulation layer having a thickness (d nm) substantially satisfying the following relationship:d=(100+170×k)±30where k is an integer not smaller than zero and not larger than 5.
In accordance with the IPS LCD device of the present invention, the thickness of the insulation layer satisfying the above relationship allows reduction of the leakage light in a slanted viewing direction and suppression of the chromaticity shift between the normal viewing direction (frontal viewing direction) and the slanted viewing direction.
The inventors conducted simulations wherein the wavelength characteristic of the transmittance was controlled in the normal viewing direction and the slanted viewing direction by setting the thickness of the insulation layer, to obtain a suitable range of the thickness of the insulation layer.
It is to be noted that if the insulation layer includes a plurality of films stacked one on another, the total thickness of the layered films should satisfy the above relationship. The insulation layer may be a SiNx film having a refractive index of around 1.8 to 2.0. Another layer may be interposed between the insulation layer and the substrate body so long as the another layer has a refractive index substantially same as the refractive index of the substrate body.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.